1. Field of the Invention
The invention relates generally to the field of electro-acoustic transducers, or loudspeakers, using planar elements, or diaphragms.
More specifically, the invention relates to a thin loudspeaker system using planar diaphragms fashioned from rigid, lightweight panels. The particular configuration allows the speaker system to be mounted directly upon a support wall, or the like, in such a way that the loudspeaker system and the wall cooperate in an acoustically advantageous manner.
The invention also relates to an improved combined stationary coil and moving magnet electromagnetic drive assembly for the lighweight planar diaphragms, utilizing state of the art magnetic material having an extremely high energy product.
2. Description of the Prior Art
From the standpoint of a design ideal, the mechanical resistance, or impedance, of the air impinging upon the diaphragm of an electro-acoustic transducer should form an appreciable portion of the total electrical impedance which the transducer presents to the electrical driving energy source. This ideal electro-acoustic transducer, then, would effect an efficient couple, or match, between the electrical energy source and the mechanical load which the air present to the acoustical wave producing diaphragm. Additionally, with a high coefficient of acoustical coupling, the performance of the transducer would become highly predictable. In other words, with the surrounding air mass comprising a substantial, stable, and frequency-independent load for the transducer, the vagaries in acoustical response introduced by transducer enclosures and spatial placement can be minimized.
Since air is a light and subtle medium, an acoustical diaphragm must engage a large number of air molecules to produce a reasonable sound level. It is apparent, further, that a planar diaphragm, which by its nature is capable of presenting a large surface area to the surrounding air, should be an efficient means for coupling to, and placing into motion, a large mass of air. Owing to its high coefficient of acoustical coupling, a large planar diaphragm need not make large and rapid excursions to create a substantial sound level. Making limited and relatively slow excursions, a planar diaphragm is able to avoid the acoustical incongruities characteristic of a conventional cone-shaped diaphragm.
Restricted by constructional considerations to a relatively small maximum size, a cone-shaped loudspeaker must make large and rapid axial excursions to produce an acceptable level of sound pressure. That is to say, since the cone diaphragm cannot directly couple a large mass of air, it must compensate by quickly displacing what air it does engage a considerable distance to reproduce sound at satisfactory levels.
As a result of this basic requirement of a large cone excursion, a number of well known electrical and mechanical problems arise with a conventional moving coil, cone-shaped loudspeaker. The speaker's moving coil, attached directly to the cone, creates a motion-related inductive reactance, or back EMF, which is directly related to the heightened distance and speed through which the coil must move each cycle. This dynamic back EMF, in turn, causes peaks and dips in speaker response which vary with overall speaker amplitude.
When the moving coil exerts translational force to the peak portion of the suspended cone diaphragm, irregularities in the cone's mechanical response occur. Unable to respond to the applied force in linear fashion, the wobbling cone creates skewed wave fronts which interfere to the detriment of a smooth acoustical response.
A more subtle acoustic deficiency is inherent with the large diaphragm excursions characteristic of cone speakers. To maintain compliance with a given input waveform, the cone diaphragm must also travel faster than a planar diaphragm, since the former is being displaced a greater distance. At high volume levels, when excursions are the greatest, the cone moves so fast that the displaced air is highly compressed, causing a veiled, but still perceptable aural distortion, or breakup. The planar diaphragm with its less drastic movement, is free from this compressive distortion of the air.
While the planar diaphragm has the potential to overcome many of the inherent deficiencies of the cone shaped diaphragm, as previously indicated, the prior art relating to planar loudspeakers has not solved several remaining problems, as will now be explained.
Planar diaphragms, as all other diaphragms, physically oscillate in response to the input waveform, producing both a front and a rear wavefront. If the rear of a planar diaphragm loudspeaker system is placed near a wall, or other reflective surface, the backwave will be returned to interfere acoustically with the front wave. This acoustic interference will produce amplitude peaks and valleys at varying frequencies, making linear response of the system impossible. Additionally, a portion of the reflected backwave will impinge upon the radiating diaphragm itself, resulting in unwanted mechanical and electrical reactances. While these adverse effects can be lessened, to some extent, by placing the system some distance from the rear wall, such placement is physically impractical or esthetically undesirable in many installations.
Most of the loudspeakers having planar diaphragms use diaphragm driving assemblies which are inherently mismatched to the source. The electrostatic driver, for instance, requires a step-up transformer having a large inductive reactance component. This substantial inductive reactance imposes both a load problem for the driving source and a limitation upon the high frequency response of the system. Thus, within the known prior art associated with planar diaphragm loudspeakers, considerable room for improvement exists both in the treatment of the "backwave problem" and in the electro-mechanical means for driving the planar diaphragm.